In the aforementioned type of oleo dampers, the orifice means has been recognized as having a great influence on the damping property. Where the orifice is of a type having a constant area, inconveniencies will be encountered in that an appropriate damping property cannot be ensured throughout the load range. When the orifice area is so determined that the damper provides a desirable damping property against a shock load which is relatively small, an adequate damping effect cannot be afforded when the damper is subjected to a large shock load. On the contrary, when the orifice area is such that an adequate damping effect is obtained against a relatively large shock load, the damper will become so stiff that a proper damping cannot be afforded against a small shock load. In case where the oleo damper is applied to a motorcycle, it cannot provide a satisfactory damping both in operation on a rough surface and on a relatively smooth surface.
In order to solve the problem, proposals have been made to provide means for changing the orifice area in accordance with the amount of the shock load. For example, in Japanese utility model publication Sho 48-39515 published on Nov. 20, 1973, there is disclosed an oleo damper having a tapered needle adapted to be inserted into an orifice aperture formed in a piston so that the orifice area is changed by the needle in accordance with the position of the piston. Since the position of the piston is dependent on the amount of shock load, the orifice area is in effect changed in accordance with the shock load. Similar devices are also disclosed by Japanese patent publications Sho 52-14831 published on Apr. 25, 1977 and Sho 52-16555 published on May 10, 1977.
The oleo dampers in accordance with these proposals are considered as having a wide range of adaptability because the orifice area is changed in accordance with the amount of shock load so that appropriate damping property is afforded throughout a wide range of shock load. However, these known dampers still have disadvantages because they have no temperature compensating means. Hydraulic oil in the oleo damper is subjected to a temperature rise in use and such temperature rise causes a decrease in the viscosity of oil. Thus, there will be a decrease in damping effect in accordance with an increase in the oil temperature.
It is therefore an object of the present invention to provide an oleo damper in which damping orifice area is changed in accordance with the amount of shock load and also with the oil temperature.
Another object of the present invention is to provide an oleo damper which can be used in a wide range of oil temperature and against a wide range of shock load.